Step 1:

Select Transactions on the sidebar menu.

Step 2:

This section shows all of your transactions (the most recent transaction will be shown at the top). Select the ellipsis (3 horizontal dots) on the top right of the transaction snapshot, then click on Open Batch.

Step 3:

Click on the Action button, then select Add Tip.

Enter the desired tip amount to the transaction, then click on Adjust Tip.